Multistage mixer-settler apparatus



July 21, 1953 v '8. v. COPLAN ET-N- MULTISTAGE MIXER-SETTEER APPARATUS lfiled Feb. 27, 1950 V s Sheets-Sheet 1 1 All/I- INVEN TOR. Ida/(r2 l ZeBrouAs/I'e' July 21, 1953 a. v. COPLAN ETAL MULTISTAGE MIXER-SETTLER APPARATUS- 3 Sheets-Sheet 2 Filed Feb. 27 1950 INVENTOR. A Ida/en .Zebraw51'e Barth): M( (an Patented July 2 1 1953 Burton V. Coplan, Troy, and Edwin L. Zebroski,

" Albany, N. Y., assignors to the United States of w America as represented by the-United States.

Atomic Energy Commission Application February a, 1950, Serial Nb. 146,422

.3 claims. (o1.,23-2'z0.5).' y

The present invention relates to mixer-settler units and in particular it pertains to multistage mixer settler units wherein flow of liquids under treatment is co-current in each stagewhile flow through the entire unit is countercurrent.

Forcertain uses it' is highly desirable to pro- H vide a mixer-settler of simple, compact construction which is capable of high efijcien'cy and adaptable to remote control. Heretofore mixer- The concept of using a pump-mix impeller was, therefore, temporarily set aside and various other extractor designs not involving the pump-mix impeller were tested over a. period of several months. Various diificulties with these units led to 'a further consideration of the pumpimpeller type of unit in an efiort to produce a machine wherein the liquid interface is'free of turbulence even when the impeller R. P. M. is high enough to give good contacting'eificiency'in the mixing stage of theu'nit.

It is an object of this invention to provide'a horizontal, multistage mixer-settler unit having high mixing efiiciency and close control of settlers employing stage co-current, overall 10 the interface in each stage despite density gradicountercurrent fiow'were unsatisfactory because ents due to mass transfer of solute between the of lack of control of the interface between the two phases and despite pressure drops due to the heavy and light liquid phases in the unit. It has flow of liquid through the apparatus. been suggested that the interface may be con- It is a further objectof this'invention to protrolled-near the center'of agiven stage of a hori 15. vide a mixer-settler unit having a definite and zontal type mixer-settler by tilting the entire maintainable flow pattern wherein the heavier unit. However the degree of tilt is affected by the phase is obliged to'traverse' a zone of high turbuflow rates of the phases, and when stirrers are I lence in passing from stage to stage, thereby used in the mixing zone, the design and speed of; eliminating channeling or bypassing the stirrer must be considered. Additionally, the It is a further object of this-invention to pro- I I degree of'tilt will vary with the relativedensities vide. a mixer-settler that can be operated horiof the phases. In view of these disadvantages zontally so" that provision for calculating and and since a horizontal unit is more convenient to, maintaining tilt of the apparatus can be elimiinstall and operate by remote control, a device nated. f I was sought which would make possible a mixer- The foregoing objects 'are achieved in the settler made up of a series of horizontally dis- "presentapparatus by the use of a centrifugal posed stages having positive control of the inpumptype impeller'to give fluid flow,'interface te face in ea Sta e and gi a reproduceable control and mixing action. This impeller is used. flow pattern through the unit. After aprelimiin combination with a 'novel arrangement of nary investigation-a mixer-settler unit was conhorizontal and vertical plates which permits structed consisting of a plurality of horizontally achievement of high mixing efliciency while f disposed stages each equipped witha stirrer-or maintaininggood interface control. impeller in each stage. The impeller disc was This invention may-be more fully understood mounted around a hollow shaft which extended by reference to the appended drawings'wherein: into the two liquid phases and acted as a pump. W Fig. 1 represents a typical mixer-settler stage, It was found that this application of a pumpim' '"portions thereof being broken away. peller could satisfactorily control-the phase in- -Fig. 2 is an elevational view partly in section terface at any desired point. Such a unitwas of a pump-mix stirrer or impeller of the type tested with a two phase aqueous-organicsolvent shown in Fig. 1. l 1 system to measure mixing efficiency. The tests 40 Fig. 3 is a plan view of the impeller shown in showed that in order to obtain a high stage mix- Fig. 2. ing efficiency of 96% it was necessary to rotate Fig. 4 is a top view of a ten stage mixer-settler the impeller at 1590 R. P. M. Itwas found that unit with the top cover removed. under conditions which gave good control of the Fig, 5 is cross section along line VV of Fig. interface location, the mixing efiiciency was poor, i, d

-' while the interface control became poor when i 5 isv cross secti0n.a1ong line v1 v of the impeller R. P. M. was made high enough to Fig. 4, r ive good mixing efficiency, p cially if the shaft With reference to Fig. 1, the singlestage 10 was not true and there was some whipping of the therein represented consists of an inletsection H shaft to contribute turbulence to the solutions. for a heavy phase such as an aqueous phase.

This is vertically separated from a mixing section I? bya horizontal plate [3 provided at the center thereof with a circular openingM to accommodate the shaft l5 of pump-impeller I6.

The mixing section l2 and the inlet section ll are separated from the settling section I! by a partition l8 having therein a vertical slot l9 communicating with the mixing section and. shielded by a baflle 20.

An inlet 2| for the heavier phase, suchas an aqueous solution, is positioned to admit liquid to the aqueous inlet section, which is located below the mixing section.

The lighter phase, for example, hexone, is admitted at the top of the mixing section through an inlet 22.

The pump-mix stirrer I6 is shown in Figs. 2

and 3 and consists of a hollow shaft connecting with four passages 23 drilled. radially in a horizontal disc 24. Eight vertical turbine blades 25 secured to disc 24 provide mixing action. The

lower end of the shaft [5 extends through the opening l4 in plate [3 and into the aqueous inlet section. Rotation of the stirrer is achieved .by any suitable means, such as an electric motor (not illustrated).

In operation, rotation of .the impeller !6 pumps liquid. from the aqueous inlet section and the settling sectionof the next higher stage into the mixing section, where it is contacted with the lighter phase, such as hexone which enters the mixer. section I2 from the settling section of the next lower stage. The mixed phases are discharged through slot 19 into the settling section ll. controlled in :a predetermined position by the location of the end of the hollow shaft of the impeller in thenext lower stage.

Section I] :is provided with two outlets. The

onefor the. heavier phase, 26,.being located at tion remote from the slot to permit maximum settling.

The interface in this section is accurately The mixer is rotated rapidly enough to have a pumped upas the aqueous phase. level breaks.

contact with thetip of the shaft. v

A. multiestage unit embodying the present. ine" ventionis shown in .Figs. 4 to 6 inclusive. :With particular referenceto Figs. .4 and-5, the unitcomprises ten stages 28 to 31 inclusive,- eachstage including an aqueous inlet section, a mixing sec-' tion, a pump-mix stirrer, and a settling section";

in all respects similar to the stage described in detailin connection with Figs. 1 to 3. The several stages are interconnected adjacent the top thereof by passages 38 to 46 which permit flow of the lighter phase from the top of a settling section to the mixing section of the next higher stage. A second series of passages 41 to connects the lower portion of the settling section of a stage with the aqueous inlet section of the next lower'stageu Fig. 6 shows a section through a typical passage, 48, of this series. The passage inlet 56 is located adjacent the bottom of the settling section of stage 36. The passage outlet 51 is placed higher but is below the horizontal plate 58 which separates the aqueous inlet section from the mixing section of stage 35.. illustratedin Fig. 4, a circular opening 62 in the Stirrer R. -P. M.:

bottom of the mixing section 6! of stage 28 provides communication between this section and the lower inlet section of this stage. The remainder of the cells are similarly constructed.

The entire unit is provided with a circumferential flange 59 and bolt holes 60 for attaching a suitable cover (not illustrated).

In the operation of a unit like that shown in Figs. l and 5, a light phase, such as hexone a solvent, may be introduced into the mixing section SI 7 of stage 28 from an outside source through a suitable nozzle (not illustrated). A heavier phase, such'as a solution of ammonium nitrate, NH4NO3, is pumped through passage 55 from the settling section of stage 29 and into the mixing stage 54 by a pump-mix impeller of the type illustrated in Figs. 1 to 3 inclusive. The two phases are thoroughly mixed in section 6!, and are discharged through slot 63 to the settling section of stage 28. The heavier phase flows from adjacent the bottom of this section through an outlet .(not. illustrated) while the lighter phase.

proceeds through passage 38 to the mixing section'of stage 29.. It will then be seenthat flow in tler is shown in the followingtableof two runs,

madewitha single stage pump-mix unit to determineefliciency as afunction of stirrer R. P. M. The .efiiciency was measured by transfer of nitric acid from 8M ammonium nitrate to pre-.. treated hexone. .usedz.

Two sets of flow rates were Flow rate 160 cc/min. hexone, 80 cc./'min. aqueous Stirrer R. P. M.: Percent efficiency Flow rate cc./min.'heacone, 40 cc./min. aqueous The present unit by using a horizontal plate separating the mixing section from the aqueous inlet section of each stage confines the mixing.

ficiency is achieved at stirrer speeds in the neighborhood of 750 R. P. M., whereas in apparatus without blades on the impeller and with'-' out the horizontal plate comparable efficiency was obtainable only with impeller speeds While theunit approximately twice as great.

Percent efiiciency may be capable of satisfactory operation without a horizontal plate, especially if the cell depth is increased, the presence of such a plate results in a definite improvement in the apparatus.

The position of the interface in the settling section of the stage preceding any given stage N relative to the tip of the stirrer shaft in stage 'N is a function of the pressure drops of the aqueous and hexone phases in flowing from one stage to the next, as well as the density differential between the aqueous and hexone phases. The smaller the density differential betweenthe two phasesand the. greater the pressure drop due to flow, the higher the interface above the bottom of the stage.

In those banks where the aqueous phase is being stripped and thus decreasing in density, the eifect of change in density from stage to stage is to lower the interface in the settling section. In those banks where the hexone phase is being stripped; the effect of change in density is to raise the interface in the settling section.

It will be seen that by means of the present invention a multi-stage mixer-settler unit has been provided'which givesmaximum eificiency separating said inlet section and said mixing section, combined stirring and pumping means in said mixing section comprising a shaft at least prising an inlet section for the heavy phase, an

inlet for said section, a mixing section above the inlet section, inlet means in the mixing section for admitting the light phase, a horizontal plate separating said inlet section and said mixing section, combined stirring and pumping means in said mixing section comprising a shaft at least the lower'portion thereof being hollow, a disc'fixed perpendicularly to the axis of the shaft, a plurality of vertically arranged turbine'blades secured to the periphery of the disc, means defining radial passages in said disc and communicating with the hollow lower portion of said shaft, said portion of said shaft extending through said plate and into said inlet section whereby the liquid level in the inlet section is maintainable at a predetermined level, a settling section communicating with said mixing section, and outlets in said settling section adjacent the top and bottom thereof for removal of the light and heavy phases respectively.

2. A mixer-settler unit for contacting a' light and a heavy liquid'phase comprising a plurality of horizontally arranged stages, each stage comprising an inlet section for the heavy phase, an

the lower portion thereof being hollow, a disc fixed perpendicularly to the axis of the shaft, a plurality of vertically arranged turbine blades secured to the periphery of the disc, means defining radial passages in said disc and communicating with the hollowlower portion of said shaft, said portion of said shaft extending through said plate and into said inlet section whereby the liquid level in the inlet section is maintainable at a predetermined level, a settling section adjacent the mixing and inlet sections, means defining. a vertical slot providing communication between the mixing section and the settling section, a baflie in the mixing section extending across the said slot means, and outlets in said settling section adjacent the top and bottom thereof for removal of the light and heavy phases 7 respectively.

3. A mixer-settler unit for contacting a light and a heavy liquid phase comprising a plurality of horizontally arranged stages, each stage comprising an inlet section for the heavy phase, an inlet for said section, a mixing section above the inlet section, inlet means in the mixing section for admitting the light phase, a horizontal plate separating said inlet section and said mixing section combined stirring and pumping means in said mixing section comprising a shaft at least the lower portion thereof being hollow, a disc fixed perpendicularly to the axis of the shaft, a plurality of vertically arranged turbine blades secured to the periphery of the disc, means defining radial passages in said disc and communicating with the hollow lower portion of said shaft, said portion of said shaft extending References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,176,899 Gordon et a1 Oct. 24, 1939 Mensing Aug. 6, 1946 

1. A MIXER-SETTE UNIT FOR CONTACTING A LIGHT AND HEAVY LIQUID PHASE COMPRISING A PLURALITY OF HORIZONTALLY ARRANGED STATES, EACH STAGE COMPRISING AN INLET SECTION FOR THE HEAVY PHASE, AN INLET FOR SAID SECTION, A MIXING SECTION ABOVE THE INLET SECTION, INLET MEANS IN THE MIXING SECTION FOR ADMITTING THE LIGHT PHASE, A H ORIZONTAL PLATE SEPARATING SAID INLET SECTION AND SAID MIXING SECTION, COMBINED STIRRING AND PUMPING MEANS IN SAID MIXING SECTION COMPRISING A SHAFT AT LEAST THE LOWER PORTION THEREOF BEING HOLLOW, A DISC FIXED PERPENDICULARLY TO THE AXIS OF THE SHAFT, A PLURALITY OF VERTICALLY ARRANGED TURBINE BLADES SEFURED TO THE PERIPHERY OF THE DISC, MEANS DEFINING RADIAL PASSAGES IN SAID DISC AND COMMUNICATING WITH HOLLOW LOWER PORTION OF SAID SHAFT, SAID PORTION OF SAID SHAFT EXTENDING THROUGH SAID PLATE AND INTO SAID INLET SECTION WHEREBY THE LIQUID LEVEL IN THE INLET SECTION IS MAINTAINABLE AT A PREDETERMINED LEVEL, SETTLING SECTION COMMUNICATING WITH SAID MIXING SECTION, AND OUTLETS IN SAID SETTLING SECTION ADJACENT THE TOP AND BOTTOM THEREOF FOR REMOVAL OF THE LIGHT AND HEAVY PHASES RESPECTIVELY. 